Background
In recent years, there has been controversy over the definition and diagnostic criteria of steatotic liver disease. The discussion mainly revolves around non-alcoholic fatty liver disease (NAFLD), metabolic dysfunction-associated fatty liver disease (MAFLD) [
1], and metabolic dysfunction-associated steatotic liver disease (MASLD) [
2]. Both MASLD and MAFLD are diagnosed using positive criteria, emphasizing the importance of metabolic dysfunction. A multicenter study from Brazil showed that the MAFLD definition and MASLD definition detect a similar population to NAFLD [
3]. MASLD criteria are superior to MAFLD criteria in identifying fatty liver in lean patients [
2]. However, MASLD is thought to potentially overdiagnose or misclassify individuals who are not at high metabolic risk. This study also reported that MAFLD performed better at detecting people who may be at greater risk for liver fibrosis and disease progression [
4]. Another study demonstrated lower all-cause mortality in the MASLD-only group than in the MAFLD-only and MASLD/MAFLD overlap groups [
5]. MAFLD is a liver disease with a total global prevalence of 39% [
6], increasing over time and including all demographic age groups [
7]. Recent studies showed that MAFLD patients had higher cardiovascular mortality than NAFLD patients [
8]. NAFLD has been considered a multi-systemic disease with various extrahepatic manifestations, including non-obesity-related diseases like sarcopenia [
9].
Sarcopenia is described as the gradual loss of muscle mass and strength with advancing age [
10], causing a high public health burden worldwide. Given the rapid pace of population aging worldwide, the prevalence of sarcopenia is likely to grow from now on. As a progressive disorder, sarcopenia predicts adverse outcomes including frailty, disability, morbidity, and mortality [
11,
12].
Sarcopenia is a multifactorial disease with contributing factors including insulin resistance, chronic inflammatory state, mitochondrial dysfunction, oxidative stress, malnutrition and inactivity [
13,
14]. Key drivers for the development of MAFLD are insulin resistance, mitochondrial dysfunction, lipotoxicity, and inflammation [
15], indicating that MAFLD may share common pathophysiological mechanisms with sarcopenia. A recent study focused on the influence of myosteatosis on liver stiffness in obese individuals with MAFLD [
16]. Assessments of sarcopenia have been proven helpful in risk stratification among MAFLD patients [
17]. However, the relationship between MAFLD and sarcopenia is unknown. Moreover, most research in this field focus on older adults. Sarcopenia is more common among older populations, but also occurs in young and middle-aged populations, as well as in early life [
18‐
20]. Given the shift of obesity to younger populations, MAFLD is also prevalent in the young and middle-aged populations [
21,
22]. Hence, it is important to detect MAFLD and sarcopenia early in life and make lifestyle changes.
Thus, we examined the relationship between MAFLD and sarcopenia in young and middle-aged adults in this study.
Discussion
In this representative cross-sectional study from NHANES 2017–2018, a robust association between MAFLD and sarcopenia was found in the young and middle-aged people. The association persisted after adjusting the potential covariates between MAFLD and sarcopenia, including sex, race, age, alcohol consumption status, smoking status and sedentary activity. Considering different degrees of steatosis, MAFLD phenotypes were positively associated with sarcopenia after adjustments. Among the MAFLD patients, a positive association was found between liver fibrosis and sarcopenia. Given the availability of detailed covariates and the sensitivity analyses, this study is expected to be reliable. We also found a non-linear relationship between MAFLD and sarcopenia in women, which was different from men.
Several studies have found an association between sarcopenia and NAFLD, and even advanced fibrosis [
36‐
39]. A cohort study found that NAFLD patients were longitudinally associated with a higher risk for sarcopenia, depicted by rapid loss of skeletal muscle mass [
36]. Another prospective cohort study revealed that sarcopenia was significantly related to the histological severity of NAFLD progression, proven by biopsy [
37]. Insulin resistance (IR) and inflammation may contribute to this association [
40]. Considering the shift to redefine NAFLD as MAFLD, more attention should be paid to this association, since MAFLD is more likely to capture patients with extrahepatic complications [
41] and relate to greater fibrosis scores [
6]. However, few research have assessed the association between MAFLD and sarcopenia, especially among the young and middle-aged populations. A previous research found that 7% of people aged 20–30 years have sarcopenia [
42], while some recent studies found that MAFLD was not limited to the elderly [
7,
43]. Similarly, our findings on the prevalence of MAFLD (47.8%, 95% CI: 44.6–51.0) and sarcopenia (8.0%, 95% CI: 6.2–9.9) further refutes the opinion that MAFLD and sarcopenia are uncommon in younger and middle-aged adults. Hence, we explored the association between MAFLD and sarcopenia, which would provide crucial suggestions for earlier disease detection and lifestyle modifications. The similar association was not observed between MAFLD and sarcopenia symptoms assessed by questionnaires. This may be because the results of the questionnaire are not as accurate as the results of the DXA scans. Since we focused on younger and middle-aged adults, the lower LSM value of 6 kPa was selected to define fibrosis. The different manifestations of different genders may be explained by differences in hormone levels, metabolic levels, and lifestyle [
44‐
46]. Researchers have demonstrated that the relationship between low muscle strength and lean NAFLD is more pronounced in males [
47]. A nonlinear relationship between MAFLD and ALM/BMI emerged in women, with a turning point in the low ALM/BMI range. This may be because women in the low ALM/BMI range have special metabolic characteristics or nutritional status. Further studies in this population are needed to explain this result.
Considering the importance of lifestyle, subgroup analyses on alcohol consumption status, smoking status and sedentary activity were conducted, to explore the possible impact of lifestyle on the relationship between MALFD and sarcopenia. Significant association was found after adjustment between MALFD and sarcopenia amongst drinkers and individuals with longer sedentary time (≥ 3 h). The relationship between liver fibrosis and sarcopenia also existed in MAFLD individuals who have these lifestyles. Even mild alcohol consumption has been considered to associate with deterioration of hepatic fibrosis among MAFLD individuals, which was consistent with our findings [
48]. Associations between sedentary behavior patterns and increased risk of NAFLD have been emphasized [
49,
50]. A systematic review and meta-analysis (mean or median age: 61.0–88.0 years) investigated that low sedentary behavior is linked to higher muscle strength and power [
51]. Among smoking-stratified subgroups, a robust association between MAFLD and sarcopenia was also found. Our research highlights the importance of lifestyle changes in young and middle-aged populations. To prevent over-adjustment, covariates did not include obesity, but we performed a subgroup analysis on the obese populations (BMI ≥ 30). There was no association with obesity, consistent with a previous study that reported association of muscle fat content with non-alcoholic steatohepatitis instead of muscle mass [
39].
We found that both MAFLD prevalence and severity was significantly associated with sarcopenia, further mediation analysis identified that CRP and HDL may mediate the impact of sarcopenia on MAFLD (
p < 0.05). The selection of mediators and confounders was determined by searching the literature. For confounders, age, sex, race, smoking status, drinking status, and sedentary behaviors are common confounding factors when studying the sarcopenia and fatty liver [
52,
53]. For mediators, multiple pathophysiological mechanisms are involved both in sarcopenia and NAFLD, including insulin resistance, chronic inflammation, cellular aging, and oxidative stress [
54]. Transcription level research between NAFLD and sarcopenia have proved that the key genes for the two diseases were enriched in the lipid metabolism pathways, illustrating the importance of dyslipidemia in both diseases [
55]. As for MAFLD, dyslipidemia including low HDL cholesterol levels occurs in most patients [
56]. Previous studies also have shown that inflammatory cytokines are considered to be important factors in promoting the progression of MAFLD. In Chinese obese patients, high serum CRP levels were associated with an increased risk of MAFLD and were positively related to the severity of hepatic steatosis and fibrosis [
57]. On the other hand, sarcopenia is thought to disrupt endocrine, metabolism and inflammation levels in the body [
58]. Many studies suggest that the development of sarcopenia is related to lipid metabolism disorders [
59,
60]. HDL values in patients with sarcopenia are often perturbed, with both increases and decreases seen in studies [
61‐
63]. The mechanisms of how skeletal muscle protects lipid and lipoprotein levels are unclear and may be associated with its importance in insulin sensitivity and glycemic control [
64]. Likewise, the development of chronic inflammation is important for the progression of sarcopenia, and can be reflected in circulating inflammatory cytokines [
65,
66]. Relevant studies suggest that high CRP values are related with the occurrence of decreased muscle strength [
67]. This study suggested a potential mediation effect of systemic inflammation and lipid metabolism abnormalities between MAFLD and sarcopenia, but the specific mechanism needs to be further studied.
This study had several limitations. First, the NHANES is a cross-sectional study, and only represent the US population. Second, NHANES utilized self-reported questionnaires for gathering variables, which might cause recall or self-reported bias. Third, liver biopsy has been considered the gold standard of fatty liver disease diagnosis, but it’s difficult to use in a large population-based survey. Hence, we used transient elastography for diagnosis [
68]. Steatosis was based on CAP and fibrosis was based on LSM. Fourth, subgroup analysis of obesity explored the impact of obesity on the relationship between MAFLD and sarcopenia. The results of the subgroups with and without obesity were similar. The role of BMI in this relationship needs to be further explored. Similar results were found in other studies [
69]. Finally, NHANES 2017–2018 lacks data on grip strength, which is important for assessing weakness in sarcopenia. Hence, the association of MAFLD and weakness still needs exploration.
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